This invention relates generally to electronic circuits, and more specifically to loss of signal detection circuits.
Electronic devices such as digital electronics often contain a number of electronic components. In order for the electronic device to function as intended, data is often transferred between or among the components. Components are often connected by, for example, a bus that allows for communication among the components. Other data propagation schemes such as wireless or optical communications may also be used.
Occasionally, a component or a connection between components may fail. When a failure occurs, a receiving device and downstream devices often lose the incoming signal. That is, rather than receiving the expected signal, a component downstream from a failure may receive no valid signal.
Should a component stop receiving the expected signal, it is often desirable for the component to modify its processing or state. For example, this may allow the equipment to generate an alert informing a user and downstream systems that the signal has been lost, to enter a power saving mode, or to otherwise modify processing performed by the device in view of a loss of signal.
Digital electronic devices often contain a valid signal at a low level, to indicate a logical low, for example. Such a valid low level signal may have an amplitude less than or only slightly higher than that of the noise in the system. Thus, it is generally necessary to accurately distinguish between a low level signal and noise to reliably detect a loss of signal (LOS).
The small currents and the close signal levels involved present difficulties in accurately detecting a LOS. Detecting and monitoring such signals may be difficult in view of process and temperature variations, and may require high power circuits, or complex circuit topologies. Moreover, instantaneous noise may result in an erroneous LOS alert if the magnitude of the noise is large enough.
For example, a buffer with variable hysteresis may be used. Such a circuit may require the creation of a sampling clock, with attendant layout, clock skew, and other difficulties. The frequency of the sampling clock may need to be changed as the data rate changes, possibly resulting in a complex and large circuit. Due to process and temperature variations, an accurate variable hysteresis is often difficult to achieve. The sampling clock and additional circuitry may also increase the power consumption of the circuit. Finally, as the circuit operates at high speed, it may treat transient or instantaneous noise as a valid signal if the noise is large enough.
In other circuits multiple buffers with fixed offsets may be used. Such circuits may include multiple high speed paths. The high speed paths may include logic that generates an LOS alert. The high speed paths and logic often increase the power requirements of the circuit. Due to process and temperature variations, the high speed paths are often difficult to match to one another. Due to difficulties in matching, post processing may need to be performed. Post processing may consume additional power and add further complexity to the circuit. As mentioned above, such a high speed circuit may also respond to instantaneous noise if it is large enough.
Thus, in view of the complications presented by process and temperature variations, power requirements, and circuit topology, implementing an accurate LOS detection circuit is often difficult.
The present invention provides a dual mixer loss of signal detection circuit. In one aspect, the invention comprises a method for detecting a loss of signal. The method includes providing an input signal to a first Gilbert mixer; providing an offset voltage to a second Gilbert mixer; the Gilbert mixers altering voltages on a plurality of output lines in response to the input signal and the offset voltage; time averaging a plurality of signals of the output lines, producing time averaged outputs; comparing voltages of the time averaged outputs; and generating an alert in response to the comparison of the time averaged outputs.
In another aspect, the invention comprises a circuit for detecting a loss of signal. The circuit includes a plurality of Gilbert mixers; a plurality of output lines, with each Gilbert mixer providing an output to at least two output lines; a filter circuit coupled to the plurality of output lines, producing a filtered output; and a comparator circuit receiving the filtered output and generating an alert signal.
These and other aspects of the invention may be more fully comprehended upon study of the following descriptions in conjunction with review of the accompanying figures.